1. Field of the Invention
The present invention relates to a method of producing finely divided fibrous cellulose particles.
More particularly, the present invention relates to a method of producing finely divided fibrous cellulose particles capable of forming an aqueous suspension thereof having a high viscosity and a high suspension stability even in a relatively low consistency.
2. Description of the Related Arts
It is known that fine fibrous particles produced by finely grinding cellulose particles have a large surface area, a high affinity to water, a high water-retaining power and a high capability of forming an aqueous suspension thereof having a high viscosity even in a low consistency thereof and excellent suspension stability, and therefore are useful as a humectant, dispersant, and thickener.
Also, it is known to produce fine cellulose particles having a large surface area by mechanically grinding cellulose fibers. In this conventional method, the cellulose fibers are coarsely divided by a roll crusher or a course crushing cutter and then finely divided by a rotary type mill known as a high speed impact crusher.
As commercially available cellulose particles produced by the mechanical grinding, Pulp Flock (a trademark, produced by Sanyokokusaku Pulp Co.) produced from parenchyma cells having a low mechanical strength, or Cellulose Powder B (a trademark, produced by Lettenmayer Brother Co.) are known.
Nevertheless, since the cellulose fibers are of a soft organic substance, it is difficult to produce satisfactorily finely divided cellulose particles by a mechanical grinding operation alone. Therefore, a method comprising a combination of a chemical dividing step and a mechanical dividing step is usually employed to obtain fine cellulose particles. Generally, the cellulose fibers are composed mainly of crystalline segments and amorphous segments and the amorphous segments are more reactive to reactants than the crystalline segments. This specific property of the cellulose fibers is utilized to provide the finely divided cellulose fibers in the conventional chemical dividing method. Namely, in this chemical method, the cellulose fibers are subjected to a reaction with a mineral acid, for example, to selectively dissolve away the amorphous segments and maintain the crystalline segments. By this chemical method, fine crystalline cellulose particles consisting mainly of crystalline segments are obtained.
In another known method, a light chemical treatment is applied to the cellulose fibers to reduce the mechanical strength of the cellulose fibers, and then the resultant chemically treated cellulose fibers are crushed by a mechanical treatment. This method is a combination of the chemical treatment and the mechanical grinding treatment, and disclosed in "Japanese Journal of Paper Technology" No. 8, pages 5 to 11, 1985, August.
The fine cellulose particles produced by the above-mentioned conventional method are widely used, and have various applications, for example, filtration assistance, rubber filler, excipient for medical tablet, suspension-stabilizer, thickener and shape-retaining agent.
Where the conventional fine cellulose particles are used as a suspension-stabilizer, thickener or shape-retaining agent in which the suspension-thicking effect, dispersion-stabilizing effect and gel-forming effect of the conventional fine cellulose particles are utilized, it is necessary to employ the conventional fine cellulose particles in a high consistency or in a large amount, because of the low affinity of the conventional cellulose particles to water. The increase in the amount of the fine cellulose particles used results in an economic disadvantage. Particularly, when used for foods, the increase in the content of the cellulose particles in the food results in a disadvantage in that the resultant food is rough and unpleasant to the touch.
To eliminate the above-mentioned disadvantage, Japanese Examined Patent Publication (JP-B) No. 57-14,771 discloses fine crystalline cellulose particles coated on the surfaces thereof with a water-soluble high molecular substance for food use. The coated cellulose particles are also disadvantageous in the high moisture-absorbing properies thereof, high rotting properties thereof when dispersed in water, or a significant reduction in viscosity thereof when heated.
It is known that the aqueous suspension of the fine crystalline cellulose particles can be homogenized by extruding the aqueous suspension of the fine crystalline cellulose particles through an orifice having a small inside diameter under a pressure of at least 200 kg/cm.sup.2 to impart a high velocity to the suspension, and striking the stream of the suspension against a hard face to rapidly reduce the velocity and applying shearing and cutting actions to the cellulose particles. When the above-mentioned steps are repeatedly applied, the suspension stability of the fine cellulose particles in water is enhanced. The resultant aqueous suspension of the fine crystalline cellulose particles exhibits enhanced suspension stability and high viscosity even in a very low solid consistency thereof. This method is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 59-120,638.
Also, U.S. Pat. Nos. 4,374,702 and 4,483,743 disclose a method of preparing microfibrillated cellulose, in which an aqueous suspension of fibrous cellulose is homogenized by extruding the suspension through a small diameter orifice so that the suspension is subjected to a pressure drop of at least 3000 psi and a high velocity shearing action followed by a high velocity deceleration impact against a solid surface
However, these high pressure homogenizing methods are disadvantageous in that the extrusion operation of the aqueous cellulose particle suspension through a thin orifice under high pressure must be repeated, and thus the treatment efficiency is low and the cost is high.
Accordingly, there is a strong demand for providing a method of producing finely divided fibrous cellulose particles capable of forming a highly stable aqueous suspension thereof having high viscosity even in a low consistency, and high production efficiency.